Means for measuring and utilizing small direct currents



Dec. 2, 1941. H. M. SCHMITT 2,264,956

MEANS FOR MEASURING AND UTILIZING SMALL DIRECT CURRENTS Filed Nov. 21, 1934 2 Sheets-Sheet l INVENTOR. HENRY M. SCHMITT BY &

A TTORNEY' 1941- H. M. SCHMITT 2,254,955

MEANS FOR MEASURING AND UTILIZING SMALL DIRECT CURRENTS Filed Nov. 21, 1934 2 Sheets-Sheet 2 i 37 FIG4 IN VEN TOR. HENRY M. SCHMITT BY 8 M65 ATTORNEY Patented Dec. 2, 1941 was run amasuamo AND u'rnrzmo EMAIL DIRECT llenryllartinschmittl'hiladelplllala assignor to'lhehrownlnstrment phia,la.,aoorporationoflennsyivania Philadel- Applieation November 21, 1984, Serial No. 754,185

33Claims.

The general object of the present invention is to provide improved means for measuring and/or utilizing for control and analogous purposes. minute direct currents such as those resultin from the voltage variations of the thermo-couples in measuring temperatures, or from photovoltaic cells. 4

One object of the present invention is to provide novel and effective means for converting a small direct current into an alternating current which may be readily amplified for measurement and other purposes. In the preferred mode of attaining this object of the invention, I utilize the source of small direct current electromotive force to create a pulsating current by conecting said source in circuit with a resistance formed by a selenium cell or other device, varying in electrical resistance with the amount of light transmitted to it, and transmit light to said device from a source of light varying in intensity with suitable regularity and frequency, and advantageously consisting of a neon glow lamp energized by alternating current of commercial frequency. The pulsating current thus created is converted by suitable inductive resistance, or other apparatus such as an ordinary transformer into alternating current which may be amplified by the use of one of various known forms of amplifiers such as the thermionic tube amplifiers commonly employed in radio circuits.

The production of alternating current from a direct current source of electromotive force in the manner described is characterized by the simplicity and eflectiveness of the apparatus required and particularly by the fact that it does not include movable mechanism parts such as are required by certain methods heretofore proposed for converting small direct currents into pulsating or alternating currents for amplification purposes. The conversion of direct current into alternating current-in the manner and for the purpose described is desirable also, because it permits of the avoidance of undesirable phase distortion and because the desirably simple and symmetrical form which the alternating current waves may have. A further advantage of this method is that the apparatus for converting minimizes or eliminates stray potentials that would themselves set up undesired alternating currents. The selenium cell can be made essentially symmetrical eliminating stray contact potentials or other sources of E. M. 1".

. A further object of the invention is to elect a novel and desirable combination of means for converting direct current into and amplifying apparatus shown in Fig. 2; and

alternating current with potentiometric measuring means to thereby produce a novel and relatively simple form of self-balancing potentiometer instrument which may follow the approved practices of the art in respect to many of its features, and may be produced, for example, by the elimination or replacement of a relatively small portion only of the parts of a potentiometer instrument of desirable commercial form.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, and the advantages possessed by lt, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 is a diagrammatic representation of current converting and amplifying means;

Fig. 2 is a diagrammatic representation of potentiometric measuring apparatus including current converting and amplifying means; and

Fig. 3 is a diagrammatic representation of a form of relay motor which may be used in the Fig. 4 is a diagrammatic representation of a modification of the arrangement of Fig. 2.

In Fig. l, A represents a variable source of light advantageously in the form of a neon lamp having its terminals connected by conductors to a source of alternating current which may be of commercial frequency, 'so that the lamp emits light varying in intensity as the current supplied by the conductors I and 2 alternately varies in direction and magnitude. An electrical resistance B varying with the intensity of light to which it is subjected, and which may be a selenium or other photoelectrical cell, receives light from the lamp A. The electrode terminals of the device B are connected in series with the primary winding C of a transformer C and a thermo-couple D or other source of direct current electromotive force of small intensity. The secondary winding 0 of the transformer C is connected to the input terminals E of an alternating current amplifying device E having output terminals E and power current terminals E. The amplifying device E may be of any usual or suitable form and in particular may be such an amplifying device including one or more thermionic tubes as is commonly employed in radio circuits and for analogous purposes to deliver alternating current at its output terminals E suitably related to, but of greater magnitude than 'the alternating current supplied to its input terminals E. As shown a suitably conversing lens F is interposed between the lamp A and the light sensitive B.

As those skilled in the art will recognize, the arrangement shown in Fig. l is characterized by its simplicity and eii'ectiveness, and in particular by the avoidance of mechanism including movable parts heretofore proposed for use in converting small direct currents into alternating currents for current amplification purposes. As those skilled in the art will recognize, the char: acteristics and proportions of the parts of the apparatus shown'in Fig. 1 may readily be made such that the alternating currents delivered to the input terminals E and delivered from the output terminals E' of the amplifier E may be suitable simple and symmetrical in wave form and desirably free from phase distortion or displacement relative to the phase of the current supplied to the terminals of the lamp A bythe s ly conductors l and 2.

While the input terminals 1!! of the amplifier E are convenientlyenergized through the transformer C, other provisions, such as those shown in Fig. 2, may be made for the energization of said terminals by the pulsating current ilow in the circuit which includes the light sensitive resistance B and is energized by the thermo-couple D or other source of a small uni-directional electromotive force.

The parts A, B, C, E and F, associated generally as shown in Fig. 1, may be combined with other apparatus in the manner shown in Fig. 2. In the arrangement actually shown in Fig. 2, however, the light source A of Fig. l is replaced by a light source AA of different form, and the transiormer C of Fig. i is replaced by a resistor C which in the circuit including the light sensitive resistance B, replaces the transformer primary of Fig. l, and which has its terminals connected to the amplifier terminals E. It is to be noted, however, that for the general purp ses of the arrangement of Fig. 2, the resistor C is the equivalent oi' the transformer C of Fig. l. and the same general results would be obtained if the resistor C of Fig. 2 were replaced by the primary C of the transformer C and the secondary C" of the latter were connected to the amplifier terminals E as in Fig. l.

The light source AA of Fig. 2 may be a neon tube but it diflers from the tube A of Fig. l in that me electrode thereof is shielded from light sensitive device B. The electrodes of the Fig. 1

light source A are disposed side by side so that light from first one and then the other impinges on cell B. In Fig. 2, however, one electrode is in the form of a relatively small plate and the other electrode is in the form of a larger disc disposed between the plate electrode and device B so that the larger disc electrode completely shields the smaller plate electrode from light sensitive device B and the latter will thereby be responsive to source A during only one half of .each cycle. The purpose oi thus shielding one electrode of source AA is to permit selective operation of a reversible motor as hereinafter described when the light source is energised from the same source as held I of motor H or from a source having the same frequency as the source for held I.

In Fig. 2 the light sensitive resistance B, the

ing to maintain a potential diil'erence between i the points 0 and 0' equal and opposite to the E. M. F. of the thermo-couple D. The potentiometer circuit 0 shown in Fig. 2

is of the split potentiometer type and comprises three branches connected in parallel, one branch including the main potentiometer slide wire re-v sistor G a second branch including balancing resistances G and G and the third branch ineluding a battery 0' or other source of current v and a resistance (II which may be adjusted as required to insure a current now of the desired magnitude through the resistor G A contact a engages the resistor a at the point g ,'th.c position or which along the length oi the resistor G is varied by the sliding adjustmentof the contact G The latter as shown is a bridging contact connecting the point p of the resistor G to a corresponding point of a slide wire resistor G" alongside the resistor G The resistor G" has one end connected by a conductor 6 to one terminal of the thermo-couple D, and is employed for the known purpose oi avoiding measurement inaccuracies due to variations in resistance to the now of current generated by the thermo-couple D which would otherwise result from variations in the relative resistances of the portions of the resistor G at opposite sides of the point 9 The second terminal of the thermo-coupie D is normally connected through a switch (3 and conductor 4 to one terminal of the cell B. The other terminal or the cell B is connected by a conductor I to the branch of the otentiometer G including the resistances G and G at the point in said branch intermediate said resistances.

with the arrangement shown in Fig. 2, the flow, and direction of flow, of current through the circult branch including the conductor I, light sensitive resistance B, resistance C conductor I, switch 0'. thermo -couple. D, conductor I, portion of the resistance '6" at the right of the bridging contact G and the latter, depends upon the relation between the voltage of the thermocouple D and the potential diilerence between the points a and a. The thermo-couple D is so connected to the potentiometer circuit that the electromotive force of the thermo-couple opposes the potential diirerence between the points a and g. The potential diilerence between the points g and 0' is increased and decreased by movement of the contact G to the right and to the left, respectively. With a suitable adjustment of thermo-couple voltage above the potential diilerence between the points a and a,- current will flow in one direction through the re'sistanoeC' and such current flow may then be eliminated by a. suitable adjustment of the bridging contact G to the right. Conversely when the voltage of the thermo-couple' falls below the potential difference between the points 9 and a. the resultant current flow through the resistance C' will be i in such direction that it may be eliminated by a suitable adjustment of the contact 0' to the left. As diagrammatically illustrated, the bridginl contact (3 is adjusted along the slide wireresistances G and G by the operation of a relay motor Hshown as having its armature shaft H threaded through the contact G, so that the latter is adjusted longitudinally of the shaft H in I to and energized from the supply conductors l and 2 which energize the lamp AA, and, as shown, supply power current to the amplifier terminals E.

For its intended use the motor H may be of the form diagrammatically shown in Fig. 3, and comprising a stator core with an opposed pair of main poles h and M, and with auxiliary poles h and h at opposite sides of the pole h, and with auxiliary poles M and W at opposite sides of the main pole ha. The armature H of the motor H has a squirrel cage winding andturns within the cylindrical space surrounded by the poles h, h and h', ho, ha, and M spaced about the armature in the order stated. The yoke portion h of the 'stator is surrounded by a main winding I connected between the motor terminals H and the auxiliary poles h, h, M and lid are surrounded by auxiliary windings or shading coils i, i", M and to which are connected between the motor terminals H.

The motor H is essentially similar to a two phase induction motor and depends, for its operation upon phase displacement of the current flowing in coils M, h", M and M, collectively and the current flowing in coil 1. The inductance of the coil I is large so that the current therein will be approximately 90 out of phase with the voltage across line l-I. When source AA is energized from the same line, as shown in Fig. 2, the resistance of cell B varies from minimum to maximum as the voltage of the line varies from maximum to minimum in either direction so that whenever current flows in the resistor Clot the potentiometer circuit it would, if both electrodes of source AA were exposed to cell B, be of a pulsating nature of double the frequency as the line frequency. As previously stated I mask one electrode of cell AA to thereby reduce the frequency of voltage pulsation in resistor C to correspond to the frequency in the line.

The amplifier output frequency corresponds, of course, to the frequency in resistor C and is approximately in phase therewith so that as a consequence of the inductive phase displacement between the current flow in coil I and the line voltage the amplifier output current will either lag behind or lead, but will be of the same frequency as the current in coil I. Whether the amplifier output current is approximately 90 behind or is approximately 90 in lead of the current in coil I depends upon the direction of current in resistor C in response to rise or fall of the voltage of thermo-couple D above or below the value for which resistor G is then adjusted. With resistor G balanced for the existing value of the thermo-couple voltage no current will flow in resistor C and motor H will be at rest. Upon any variation of the thermo-couple voltage from according to the direction of phase displacement of the amplifier output from the energizing current for coil I.

As previously indicated the potentiometer instrument shown diagrammatically may follow theapprovedpracticeoftheartinrespectto all of its features other than the mechanism employed to give balancing adjustment movements to the slide wire contact 6'. For example. the invention is well adapted for use in the well known type of potentiometer known as .the "Brown" potentiometer sold and manufactured by the Brown Instrument Company, and the general construction and operation of which is illustrated and described in the application for patent. Ser. No. 546,290, filed June 23, 1931, by Harrison, Grauel and Kessler, and issued March 14, 1939, into Patent 2,150,502. The Brown potentiometer as heretofore constructed includes a mechanical relay clutch mechanism, through which a shaft analogous in function to the shaft H of Fig. 2 is periodically rotated as required for potentiometer rebalancing, and a galvanometer and clutch control mechanism through which the deflection of the galvanometer controls the action of said clutch mechanism. In making use of the present invention, said clutch and clutch control mechanisms and the galvanometer of the Brown potentiometer, may be replaced by the motor H and the means shown in Fig. 2 for energizing the terminals of that motor. While the changes referred to do not require modification of major portions of the Brown potentiometer which includes recording apparatus and in many cases control provisions, the changes eliminate relatively expensive and complicated galvanometer and relay clutch mechanism parts and give increased sensitivity both in respect to the small magnitude of the thermo-couple voltage change which can be accurately measured and the quickness of response of the instrument to changes in,

that voltage. Thus contact G or other means driven by shaft H may be arranged to move a recorder pen or indicator across a moving chart or suitable scale to record or indicate the-quantity measured.

It will be apparent that motor H may be employed to operate a control valve or the like for governing the fuel supply to a furnace, for example, to which thermocouple D is responsive, in lieu of or in addition to operating the shaft H, or another motor, desirably operated together with motor H, may be so employed. For example, as shown in Fig. 4, a furnace DD, to the temperature of which the thermo-couple D responds, is supplied with the fuel, which may be oil,

through a supp y pipe K at a rate determined by the adjustment of a fuel supply valve K adjusted by a motor HH. The latter may be exactly like the motor H, and in Fig. 4 is shown as having its terminals H and H connected in parallel with the terminals H and H respectively, of the motor H to the amplifier output terminal t and tothe supply conductors land 2, respectively. Obviously, however, the motors H and HH may be connected in series instead of in parallel. The mechanical connection of valve K to motor HH in Fig. 4 is such as to increase and decrease the fuel supply as the temperature to which the thermo-couple D is responsive, drops below or rises above a predetermined normal.

The motor adjusting the fuel valve K may be operated by or in accordance with the output of the amplifier E of apparatus including the ampliiler E and thermo-couple D of Fig. 2, but not I thattemperature.

With the apparatus thus far described, upon a thermocouple or amplifier tube burn out, or

other failure of the control apparatus, occurring when the fuel valve adjustment is such as to supply more fuel to the furnace than is thereafter required, the furnace temperature may become excessive, with resultantlniury to the furnace and its: contents. before the failure is detected and the fuel valve K manually adjusted to suitably reduce the heat supply to the furnace. To guard against this objectionable possibility, suitable safety means are preferably incorporated in .the control apparatus, and in Fig. 4, I have illustrated a desirable form of safety means for the purpose, whereby in case of failure of the control apparatus the motor which directly or indirectly adjusts the fuel valve K will automatically operate to close that valve. The safety means illustrated in Fig. 4 comprises a transformer T having its primary connected to the supply conductors I and 2 and having its secondary so connected between the amplifier output conductors I that the motor HH will be energized by the transformer to operate in the direction to give a closing adjustment of the fuel valve K when the secondary voltage of the transformer T exceeds the voltage of the amplifier output terminals I.

With the transformer '1 connected as shown in Fig. 4 between the conductors I of such apparatus as is shown in Fig, 2, the relation of the contactorG'andresistorG' mustbesoadjustedthat with the transformer T out of service the apparatus would tend to maintain the controlled temperature at'a value higher than the predetermined value. In consequence. when the thermocouple voltage D is normal and the contactor G' is in its normal value position, the potentiometer circuit'will not-be in balance, as in the arrangement' shown in Fig. 2, but will be unbalanced, and the unbalance should then be such as to produce an amplifier output voltage equal and opposite to the output or secondary voltage of the transformer '1'. Then the motor; H and HH of Fig. 4 will operate in one direction or the other. or will remain stationary. accordingly, as the temperature of the thermocouple D is above, below or at its normal value, as they do in the arrangement shown in Fig. 2, so long as the thermocouple and amplifier circuits are operative, but, on a failure of the thermocouple or amplifier circuit which deenergises the amplifier output terminals E, the motors H and HH. of Fig. 4 will operate in the direction required for the closure of the fuel valve 1'.

In the use of the invention in the form shown in Fig. 2, it may be desirable to eliminate. effects due to stray alternating current voltages which may be impressed on the circuit including the thermocouple D, resistance B and resistance C masco s eifects by the defiecm O,powerlinesorthelihenearthe turbing eflects, some. at least, of which have the filter L comprisingsuitable condensers'and impedancesconnectedintheseriesparallelcircuit shown between the circuit conductors I and l in Fig. 2.

With the arrangement shown in Fig. 2, the potentiometer circuit may be recalibrated from timetotimebyadjusianentofthe resistanceG' insaidcircuitasrequiredtomaintain the current flow through the resistor G approximately constant notwithstandim variations in the voltage of the battery '0 or other changes in conditions. To facilitate such recalibrating operations, the switch may be shifted into its dotted line position in which the thermocouple D is open circuiied and the light sensitive resistance B is connected in series with a standard cell G and a suitable resistance G" in shunt with the resistance 0. With the resistance G3 and G" suitably proportioned relative to other elements of the potentiometer circuit, the latter may then be properly recalibrated by adjustment oftheresistanceGasmaybe-requiredto eliminate current fiow through the light sensitive resistance B when the contact G occupies a predetermined position in which it is then maihtained by the apparatus as'a result of thefact that the potential drop through the resistance 0 is then equal and opposite to the electromotive force of the standard cell (3'.

The contacior a may be adjusted along the resistor G to recalibrate the energizing circuit of the potentiometer manually, or automatically as by operatively connecting it to the motor H during the recalibration periods. Advantageously,'however,- the instrument may include a special recalibrating motor HB which may be similar in principle to the motor H and which hasitsterminalsHconnectedtothe powerline conductors i and I. and which has its terminals H3 connected to the amplifier output terminals I throughout each recalibration period. In the arrangement diagrammatically shown in Fig. 2, a switch J is provided to disconnect the terminals H of the motor H from the conductor I and connect the latter to the'terminals H of the motor H3 at the beginning'ofeach calibration period and to restore the original condition in which the motor BB is disconnected from, and the motor I! is connected to the conductors I at the end of each necting theleverG'toonearmofabellcranklever .l pivoted at J and having its second arm connected by a link .1 tothe switch J. Asshown inFig.2,theshaftH ofthemotbrHBhssa threaded portion passing through a threaded aperture in the carrler for the contactor a so that the rotation of the motor shaft H" in one directionortheotherwillmovethecontactg" in one direction or the other along the resist-- ance G.

Means may be provided for periodically actuatas a result of induction from the potentiometer ing switches a and J at suitably. mum

tervals. For example, when, as is usual in a recording instrument the latter includes a con stantly operating timing motor for advancing the record chart, the timing motor may oscillate the lever J through a suitable cam means but no such means are illustrated or need to be described herein as they form no part of the invention claimed herein.

While in accordance with the provisions of the statutes, I have illustrated and described the best form of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims and that in some cases certain features of my invention may be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

I 1. The combination with a source of small unidirectional electromotive force, of potentiometer measuring means to which said source is connected and comprising an alternating current motor reversibly rotatable to adjust said means as required to create a potential difference therein normally balancing said electromotlve force, said motor comprising a main field winding and shading field windings and rotating when alternating currents flow through said windings in a direction depending on the phase relation of said chrrents, means for causing current flowing through said source to pulsate, means continuously connected to the terminals of said shading field windings and energized by the pulsating current to deliver alternating current displaced in phase from said current fiow in one direction or the other accordingly as the direction of fiow of said pulsating current is in one direction or the other to said shading field windings and separate means continuously connected to the terminals of said main field winding for supplying alternating current to the said main field windmg.

2. The combination with a source of small unidirectional electromotive force, of potentiometer measuring means to which said source is connected and comprising an electric motor reversibly rotatable to adjust said means as required to create a potential difference therein normally balancing said electromotive force, said motor comprising a main field winding and shading field windings and rotating when alternating currents fiow through said windings in a direction depending on the phase relation of said currents, means including alternating supply means for causing current fiow through said source to pulsate, and amplifying means continuously connected to the terminals of said shading field windings and energized by the pulsating current to deliver alternating current displaced in phase from said current fiow in one direction or the other accordingly as the direction of fiow of said pulsating current is in one direction or the other to said shading field windings, the main field winding having its terminals continuously connected to said alternating current supply means and receiving energizing current therefrom.

3. In a system for producing control effects in accordance with the value of a control condition, a single device responsive to said condition and adapted to produce an electrical current of one polarity or of opposite polarity depending upon the value of said condition, a reversible electric control motor adapted to operate under control of said electrical current in one direction or the other depending on the value of said condition and means adapted to operate said motor in one oi. said directions upon termination of the control 01' said motor by said device.

4. In a control system for a process having an inoperative condition and having an operative condition maintained by the application of a control agent, a reversible electric motor creating control effects by its operation in accordance with its direction of operation to thereby control the application of said agent, energizing means external of said motor and independent of said condition constantly tending to energize said motor for operation in a direction to prevent application of said agent and adjust said process to its inoperative condition, and means normally responsive to a control condititon for subjecting said motorv to an energizing efiect tending to rotate said motor in the opposite direction.

5. In a system for a process having an inoperative condition and having an operative condition maintained by the application of a control agent normally operative to produce control effects in accordance with the value of a variable control condition to thereby control the application of said agent, a reversible electric control motor, electrical energizing means external of said motor and independent of said condition for subjecting said motor to an energizing effect operating the motor in a direction to prevent the application of said agent and adjust said process to its inoperative condition, and means normally operative in response to said condition, for wholly or partially neutralizing said effect, or for subjecting the motor to an energizing effect operating the motor in the reverse direction accordingly as the value of said condition is at, or at one side or the other of a predetermined value of said condition.

6. In a system for controlling a temperature a reversible electric motor controlling the heat supplied to maintain said temperature, energizing means external to said motor constantly tending to operate the motor in the direction to reduce said heat supply and means responsive to said temperature for subjecting said motor to an energizing effect which tends to operate the motor in the direction to increase the heat supply and which increases and decreases as said temperature decreases and increases, whereby said motor will operate in one direction or the other or will remain stationary accordingly as said temperature is above, below, or at a predetermined value when said last mentioned means is operative, and when it becomes inoperative, said motor will operate in the first mentioned direction.

7. In a temperature control system normally operating to maintain a controlled temperature at a predetermined value, means for creating an electromotive force which increases in magnitude as the controlled temperature diminishes below a temperature higher than said predetermined value, a reversible electric motor tending to increase and decrease said temperature according to its direction of operation, and means for subjecting said motor to an energizing eifect which is the resultant of the first mentioned electromotive force and an opposing constant electromotive force equal to the first mentioned force when said temperature is at said predetermined v iagsaid motor operating in the direction to increase or decrease the controlled temperature accordingly as the firstmentioned force exceeds or is less than said opposing force.

8. In a temperature control system, a heat supply regulatnr having a normal position, a forward position, and a reverse position, and regula-- tor controlling means, including a reversible electric motor and a thermionic discharge device controlling said motor through which said regulator is normally operated to increase and decrease said supply in accordance with variations in a control temperature,and including means operative upon failure of said'device to operate said regulator to decrease the heat supply.

9. In a system for controlling a variable condition, a condition regulator having a normal position, a forward position, and a reverse position, and regulator controlling means, including a reversible electric motor and thermionic means controlling said motor, normally operating said regulator to vary said condition in accordance with its value, and means operative upon failure of said thermionic means to operate said regulator independently of the value of said condition. 10. Electrical apparatus for measuring a variable condition, comprising a source of uni-directional electromotive force the magnitude of which is controlled by said condition, an electrical circuit electrically connected with said source including a relatively fixed resistor, a variable resistor for causing said electromotive force to produce a pulsating current flow of predetermined frequency in said circuit, means associated with said fixed resistor for amplifying pulsating current fiow through the latter and filter means in said circuit for rendering extraneous currents substantially non-pulsating in said resistors.

11. Electrical apparatus for measuring a variable condition comprising a source of uni-directional electromotive force the magnitude of which is controlled by said condition, an electrical circuit electrically connected with said source including a relatively fixed resistor, a variable light sensitive resistor for causing said electromotive force to produce a pulsating current flow of predetermined frequency in said circuit, means for transmitting light regularly varying in intensity to said light sensitive resistor, means associated with said fixed resistor for amplifying the pulsating potential drop in the latter and filter means in said circuit for rendering extraneously produced currents substantially non-pulsating in said resistors.

12. The combination with a furnace and responsive element thereinof electrical temperature measuring apparatus comprising a source of unidirectional electromotive force the magnitude of which is controlled in accordance with the temperature of said furnace, an electrical circuit electrically connected with said source including a relatively fixed resistor,.a variable resistor for causing said electromotive force to produce a pulsating current flow of predetermined frequency in said circuit, means associated with said fixed resistor for amplifying the potential drop in the latter and filter means shunted about said resistors for opposing current flow therethrough of uni-directional electromotive force the mag- 76 due to electromotive forces extraneously induced in said circuit.

14. Apparatus for measuring a variable condition comprising means for producing a uni-directional electromotive force of a magnitude varying with the magnitude of said condition, and cooperating electrical apparatus electrically connected with said means energized by said force.

to create pulsating electrical currents of predetermined frequency and filter means electrically connected between said means and said apparatus to eliminate the efiect of extraneously induced currents in said apparatus of the same frequen as the first mentioned currents.

15. Apparatus for measuring a variable condition including means for producing a uni-directional electromotive force of magnitude varying with the magnitude of said condition, an electrical measuring circuit on which said electromotive force is conductively applied including an impedance, means for causing said electromotive force to produce a pulsating current fiow of predetermined frequency. through said impedance, means associated with said impedance for amplifying pulsating current fiows through the latter, and a condenser connected in said electrical measuring circuit to render currents extraneously induced in said electrical measuring circuit including currents of the same frequency it is desired to amplify substantially non-pulsating in said impedance.

16. The combination of claim .15 in which said second mentioned means includes a variable impedance and means to vary said impedance at a regular frequency.

17. The combination of claim 15 and addition- 111' impedance means electrically associated with said condenser.

18. Apparatus for measuring a variable condition including means for producing a unidirectional electromotive force of magnitude varying with the magnitude of said condition, current responsive means to indicate the magnitude of said electromotive force and including means for causing said electromotive force to produce a pulsating current flow of predetermined frequency and for amplifyin said pulsating current, a connection for applying said electromotive force to said current responsive means, and a condenser included in said connection to render currents extraneously induced in said connection including currents of said predetermined frequency ineflective to affect the indication of said current responsive means.

19. The combination of claim 18 wherein said connection also includes an impedance.

20. In combination with an alternating current motor comprising a stator and a rotor and two sets of shading coils, means for causing the electromotive forces induced in one set of shading coils to oppose the electromotive forces induced in the other set of shading coils whereby the rotor is stationary, and means comprising an amplifier electrically associated to said shading coils for assisting the electromotive forces induced in one of said sets of shading coils and for opposing and overcoming the electromotive forces induced in the other of said sets of shading coils to cause rotation of the rotor in either desired direction.

21. In combination with an alternating current motor comprising a stator and a rotor and two sets of shading coils, means for causing one set of shading coils to oppose the other set in electrical effect whereby the rotor is stationary, a control element, a second control element remotely located therefrom, means comprising a normally balanced electrical circuit extending from one of said control elements to the other, and means responsive to unbalance of said circuit for operating said first control element to restore said balance, said last mentioned means including means for applying to said shading coils electromotive force which assists the electromotive forces induced in one of said sets of shading coils and which opposes the electromotive forces induced in the other of said sets of shading coils, to cause rotation of the rotor in either desired direction.

22. In combination with an alternating current mo or comprising a stator and a rotor and two shading coils, means for preventing the shading coils from causing rotation of the rotor, said means including means for causing one of said shading coils to oppose the other of said shading coils in electrical effect, a control element, a second control element remotely located therefrom, means comprising a normally balanced electrical circuit extending from one of said control elements to the other, and means responsive to unbalance of said circuit for operating said first control element to restore said balance, said last mentioned means including means for applying to said shading coils electromotive force which assists the electromotive force induced in one of the said shading coils and which opposes the electromotive force induced in the other of said shading coils, to produce torque upon the rotor in either desired direction.

23. In combination withan alternating current motor comprising a stator and a rotor and two shading coils, means for preventing the shading coils from causing rotation of the rotor, said means including means for causing one of said shading coils to oppose the other of said shading coils in electrical effect, and means comprising an amplifier electrically associated to said shading coils for applying to said shading coils electromotive force which assists the electromotive force induced in one of said "shading coils and which opposes the electromotive force induced in the other of said shading coils, to produce torque upon the rotor in either desired direction.

24. In combination with an alternating current motor comprising a stator and a rotor and two shading coils, means for preventing the shading coils from causing rotation of the rotor, said means including means for causing one of said shading coils to oppose the other of said shading coils in electrical effect, a control element, a second control element remotely separated from the first control element, and adapted to be operated by said motor, a normally balanced electrical circuit extending between said control elements, and means including an amplifier responsive to unbalance of said circuit, for causing said motor to operate the second control element to restore the balance of said circuit, said last mentioned means including means energized by the amplifier for assisting the electromotive force induced in one of the said shading coils and for opposing and overcoming the electromotive force induced in the other of said shading coils, to produce torque upon the rotor in either desired direction.

25. The combination of claim 24 wherein the means for causing one shading coil to oppose the other shading coil is adapted to prevent fiow of current in said shading coils and includes means for causing' the electromotive force induced in one of said shading coils to oppose the electromotive force induced in the other of said shading coils.

26. In combination, a controlling element variable in condition, a controlled element variable in condition, means controlled by the relative condition of said elements for creating variations in electrical potential, an amplifier controlled by the variations in potential produced by said first mentioned means, and an alternating current motor having shading coils supplied with energy produced by said amplifier, for adjusting said controlled element.

27. The combination with a potentiometer measuring circuit including a source of electro-- motive force to be measured, a second source of electromotive force, means including a resistor to oppose said electromotive forces to derive a differential electromotive force, means to cause the differential current produced in said resistor by said differential electromotive force to fluctuate, means to amplify said fluctuating current, said resistor adapted to be automatically adjusted under control of said amplified fluctuating current to balance said second source against said first mentioned source during normal measuring periods, a calibrating resistor adapted to be adjusted during recalibrating periods alternating with said measuring periods to compensate for changes in'the magnitude of said second source, of automatic means for adjusting said calibrating resistor continuously during said recalibrating period until the recalibrating adjustment is completed, said automatic means adapted to be controlled by said amplifying means.

28. The combination of claim 27 wherein said automatic means for adjusting said calibrating resistor comprises a reversible electric motor mechanically connected to said recalibrating resistor and operating continuously during each recalibrating period until the recalibrating adjustment is completed.

29. The combination with a potentiometer measuring circuit including a source of electromotive force to be measured, a second source of electromotive force, means including a resistor to oppose said electromotive forces to derive a differential electromotive force, means to cause the differential current established in said resistor by said differential electromotive force to fluctuate, means to amplify said fluctuating current, a reversible electrical motor controlled by said amplified fluctuating current for automatically adjusting said resistor to balance said second source against the first mentioned source during normal measuring periods, a recalibrating resistor adjustable to compensate for variations in magnitude of said second source, and automatic means including a second reversible electrical motor adapted to be controlled by said amplifying means for adjusting said second resistor continuously until the recalibrating adjustment is completed during calibrating periods alternating with said normal measuring periods.

30. Apparatus for measuring a variable condition including means for producing a unidirectional electromotive force of magnitude varying 31. Apparatus for measuring a variable condition including means for producing a unidirectional E. M. F. of magnitude varying with the magnitude of said condition, current responsive means to indicate the magnitude of said e1ec tromotive force and including means for causini said electromotive force to produce a pulsatin current fiow of predetermined frequency and f0: amplifying said pulsating current, a connectior between said first mentioned means and said current responsive means to apply said electromativ'e force to said current responsive means, and electrical reactance means connected in said connection in series with said first and second mentioned means to render currents extraneously induced in said connection including currents oi said predetermined frequency ineffective to affect the indication of said current responsive means.

32. The combination of claim 31 and capacitive reactance means associated with said electrlcal reactance means.

33. The combination of claim 31 wherein said electrical reactance means includes an inductive reactance.

HENRY M. SCHMITT. 

